<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }

        .header{
            height: 100px;
            background-color: yellow;
        }
        .nav{
            background-color: red;
            width: 500px;
            height: 50px;
            margin: 0 auto;
            /* 粘性定位 */
            position: sticky;
            top: 0px;
            
        }
        .body{
            height: 1000px;
            background-color: lawngreen;
        }

    </style>
</head>
<body>
    
<div class="header"></div>
<div class="nav"></div>
<div class="body"></div>
</body>
</html>